I've been very fortunate to travel with some like minded souls every two years on dedicated battlefield tours since 2013. These have been ten+ day trips with some superlative guides. For example, my first trip was to Kursk during the 70th anniversary of the battle with Valeriy Zamulin who had been the long term curator of the Kursk museum and author of a range of books on the battle including 'Demolishing the Myth: The Tank Battle at Prokhorovka'. The trip covered not just Kursk, but Belgorod, Orel, Tula and Moscow.
In 2015, we couldn't return to Russia due to the Ukraine (the team had done Stalingrad, St Petersburg & Crimea in earlier trips), so we did Market-Garden and covered all the key battles in both the lead up and beyond the airborne drops. All the usual locations (Arnhem, Nijmegen etc) but also Antwerp, Ardennes in Belgium and beyond.
in 2017 the itinerary covered Salerno, Naples, Anzio, Cassino and Rome. In September of this year (2019) we visited Greece and Crete and covered both the ancient battles at Thermopylae, Marathon and Platea as well as various actions from WW2. For example, we covered the fighting in Greece down to the Corinth Canal and beyond and nearly a week covering the German invasion of Crete. Tim Saunders who wrote 'Crete: The Airborne Invasion 1941' was our amazing guide for this leg.
Separate to the above tours, I did two weeks in 2014 in Vietnam with a number of veterans covering Hanoi, Dien Bien Phu, DMZ, Hue, Danang and Saigon. And I am very fortunate to be going to the Falkland Islands in January (2020) guided by a veteran who fought there with 3rd Para in 1982. Definitely off the beaten track!
Beyond the above, I have separately done St Petersburg, the WW1 Battlefields in Champagne (France), Normandy and where I live, Singapore & Malaysia (Malaya). I have also been in various military museums in places such as Hong Kong, Tokyo and Canberra to name a few.
Yet to do any American Civil War Battlefields being an Aussie living in Asia, but they are on the list!